DNA rearrangements in MPC-11 immunoglobulin heavy chain class-switch variants

Immunoglobulin heavy chain class switching has been observed in vitro. In the IgG2b-producing MPC-11 mouse myeloma cell line, IgG2a-producing cells arise at a high frequency. In some cases, switch variants producing normal-sized (Mr 55,000) gamma 2a heavy chains have arisen spontaneously from a mutagen-induced "intermediate" (ICR 9.7.1) that produces an unusually large (Mr 75,000) heavy chain. Other switch variants have been isolated directly from the parent cell line. The expressed and unexpressed gamma 2b genes of MPC-11 can be distinguished in restriction endonuclease digests of total genomic DNA so that DNA rearrangements detected in MPC-11 variants can be directly associated with one or the other of these two genes. We describe here DNA rearrangements occurring on the expressed heavy chain chromosome of several MPC-11 gamma 2a switch variants and on the expressed chromosome of the ICR 9.7.1 intermediate. Our data indicate that all of these variants express the parental heavy chain variable region (VH) gene, supporting previous protein studies. We provide mapping data for the expressed gene of both ICR 9.7.1 and one of the IgG2a-producing variant cell lines (ICR 9.9.2.1) derived from it and discuss the advantages of an in vitro switching system for examining the dynamics of the immunoglobulin heavy chain class switch.